1. Field of the Invention
The present invention relates to techniques for controlling wireless communications, and, more specifically, to a wireless base station and a method of controlling wireless communications that allow switching between automatic repeat request (ARQ) modes according to the type of traffic.
2. Description of the Related Art
A hybrid ARQ is a technique wherein an error-correcting technique is added to existing ARQ functions (automatic error detection and retransmission request). In the hybrid ARQ, an error-correcting code is transmitted in advance to perform error correction, making it possible to reduce the number of packet retransmissions.
There are two types of schemes in the hybrid ARQ as shown in FIG. 1. One is called synchronous ARQ, while the other is called an asynchronous ARQ.
The synchronous ARQ, as shown in (a) of FIG. 1 is a scheme in which retransmission is performed at predetermined retransmission timings. For example, assuming a packet roundtrip time (RTT: Round Trip Time) of 6 TTIs (Transmission Time Intervals), when the transmit packet in a TTI segment of #0 (S00) is detected as an error, it can be retransmitted only at S10 and S20, each corresponding to an occurrence of #0 every 6 TTIs. In other words, retransmission is performed at frames, each of which has a number which is an integer multiple of the RTT.
The synchronous ARQ has its transmission timings predetermined. Therefore, the process number, which shows the original transmit packet for the retransmit packet, does not have to be transmitted as a control bit, advantageously allowing a reduced overhead. Moreover, the process is advantageously easy.
However, as the retransmission timings are constrained, there is a disadvantage that the flexibility of scheduling in allocating transmit data to TTIs is small. Moreover, if transmission after 1 RTT is not successful, waiting for retransmission at least until after 2 RTTs is needed, possibly causing an increased delay.
The asynchronous ARQ as shown in (b) in FIG. 1 is a retransmission scheme wherein a retransmit packet can be transmitted at 1 RTT or anytime after 1 RTT, so that the retransmission timings are not predetermined. For example, when the transmit packet at #0 (A00) is detected as an error, the retransmission may be performed at A10 or any TTI after A10 (A10 to A25). The asynchronous ARQ is used in HSDPA (for example, see 3GPP TS25.212, “Multiplexing and channel coding (FDD)”).
Advantageously, the asynchronous ARQ has great scheduling flexibility. Moreover, if transmission after 1 RTT is not successful, it is not necessary to wait until after 2 RTTs. For example, transmission is possible after an (1 RTT+1) segment, so that the likelihood of the delay increasing is small. On the other hand, the process disadvantageously becomes complex. Moreover, the process number needs to be transmitted as control bits, resulting in a disadvantage of increased overhead.
On the other hand, for allocating a wireless resource to an initial-transmit packet, persistent scheduling is known, which allocates wireless resources in constant periods and in a predetermined pattern, as shown in FIG. 2. The persistent scheduling is effective in applications such as real-time voice communications, etc., in which packets arrive at constant speed (ie., at constant intervals), communications are conducted at low rate, and overhead reduction is needed.